Field of the Invention
The present invention relates to a process for producing a target intended to be used in deposition processes carried out in a vacuum or in an inert or reactive atmosphere, especially by magnetron sputtering or by ion beam sputtering.
According to another aspect of the invention, it also relates to a molybdenum-based target possibly obtained by implementing said process and to the use of such a target for the purpose of obtaining films based on the material sputtered from said target, and also to a composition of the compound for producing said target by the process according to the invention.
Description of Related Art
Various techniques for manufacturing targets, including certain powder forming techniques, are known. Thus, the targets in question may result from a casting process or a powder sintering process followed by forming operations, often hot forming, and then assembly on a support, or direct assembly of sintered segments, or less conventionally a technique of thermal spraying and more particularly a plasma spraying technique.
These targets are intended to be used in processes commonly employed on an industrial scale for thin film deposition, especially on a glass substrate, such as for example the magnetron sputtering process. In this process, a plasma is created in a high vacuum close to a target comprising the chemical elements to be deposited. The active species of the plasma, by bombarding the target, tear off said elements, which are deposited on the substrate, forming the desired thin film.
In the specific case of a target intended for depositing molybdenum, a nonreactive deposition process is used in which the plasma is composed only of a sputtering gas, preferably a noble gas of the Ar, Kr, Xe or Ne type. This process is implemented for large substrates and may enable thin films to be deposited on substrates, for example flat glass sheets with sides of more than 6 m in length.
These targets have a planar geometry or a tubular geometry.
Planar targets have the advantage of being able to be integrated in cathodes of relatively simple architecture compared with cathodes dedicated to rotary targets, which are much more complex. However, planar targets have a utilization factor which is generally 50% or less, which is not the case for rotary targets that have a utilization factor substantially greater than 50%.
In the specific case of thin molybdenum films, molybdenum being a particularly expensive metal, it is preferred to use rotary targets of cylindrical geometry, as described in the U.S. Pat. No. 4,356,073 since these targets have a material yield (representing the proportion of sputtered material relative to the amount of material available on the target for producing a thin film) of greater than 70%, preferably greater than 75%. However, various other magnetron target geometries are known: planar (disk, square, rectangular) geometries and the invention is also applicable to geometries other than cylindrical ones.
The following literature data for pure molybdenum are given below:                density: 10.28 g/cm3;        thermal expansion: 4.8×10−6 K−1;        Young's modulus: 324 N/mm2;        electrical resistivity: 5.34 μohms·cm;        thermal conductivity: 139 W/mK;        melting point: 2630° C.        
Furthermore, there are also other vacuum processes for depositing molybdenum other than magnetron sputtering using a target: these include laser sputtering (laser ablation using a pulsed or continuous laser) and ion beam sputtering for example. These processes may also benefit from the use of a target according to the invention.
As regards more particularly molybdenum magnetron targets or those made of other refractory metals, many inventions have been filed relating to the following processes and forming the subject matter of the patent applications listed below:                Patent applications EP 1 784 518, US 2008/0193798 and WO 2006/041730:        
Pressing then sintering of an ingot or a preform (under a pressure of 200 to 250 MPa and at a temperature of 1780 to 2175° C.) followed by hot forming (at about 900° C.) of this preform by rolling or extrusion or forging. Generally, this process also includes a heat treatment in hydrogen or a reducing atmosphere in order to reduce the oxide content in the target and optionally a stress relaxation annealing treatment.                Also known, from patent application WO 2006/117145, is the complete or partial construction, or restoration, of targets by cold spraying, which consists in spraying a gas/powder mixture at supersonic velocity, the powder not being in the molten state, thereby differing from the thermal spraying processes.        
Although the above documents also cover the production of targets having various compositions using these methods, pure molybdenum targets usually have the following properties:                purity: >99.95%;        density: >95% of the theoretical density;        fine-grained microstructure.        
Targets having these characteristics are sputtered so as to obtain thin films that are used for example as electrodes for photovoltaic applications based on an active material belonging to the chalcopyrite family (CIS or CIGS for example). Molybdenum provides a good compromise between electrical conductivity (less than 30 μohms·cm), temperature resistance (refractory properties: melting point: 2610° C.) and high selenization resistance. This is because molybdenum has a higher resistance to the selenium-rich atmosphere used during the CIS or CIGS deposition step, molybdenum reacting on the surface with selenium to form an MoSe2 passivating layer without losing its electrical conduction properties, or else for TFT (thin film transistor) applications that require extremely low defect (“pinhole”) densities. Maximum pinhole densities of 500/m2 with a size between 1 and 5 μm may especially be mentioned. Such quality levels can be achieved only if the sputtering process is devoid of any electrical instability of the arcing type. This is especially possible when the target has significantly no porosity, i.e. with a density of at least 90%.